In recent years, a MIMO (Multiple-Input and Multiple-Output) transmission scheme is utilized in a radio communication field. The MIMO transmission scheme accomplishes high-speed and high-quality signal transmission by using multiple antennas, both at a transmitter and at a receiver, to execute transmission and reception.
In order to achieve higher-speed signal transmission and interference reduction, a Massive-MIMO transmission scheme is under study (e.g., Patent Document 1). In the Massive-MIMO transmission scheme, a large number of antenna elements (e.g., 100 elements or more) are used in a high frequency band (e.g., 10 GHz or higher) with which downsizing of antennas and securing of a wide bandwidth are possible.
In Massive-MIMO, advanced beamforming (BF) can be accomplished using a larger number of antenna elements compared with conventional MIMO. Beamforming is a technology in which the directivity and/or shape of beams (a transmission beam corresponding to a transmitting antenna or a reception beam corresponding to a receiving antenna) are controlled by controlling multiple antennas elements. In MIMO, because the phase and the amplitude can be controlled for each antenna element, the degree of freedom in controlling beams increases with the number of antenna elements that are used.
The beamforming that is performed on a transmission signal can be mathematically expressed as multiplication of a vector that indicates the transmission signal by a beamforming weight matrix (hereinafter, may be referred to as a BF weight matrix). A BF weight matrix can be expressed as a matrix that includes multiple beamforming weight vectors (hereinafter, may be referred to as BF weight vectors) as components. Hereinafter, the term “BF weight” may be used as a generic term for a BF weight matrix and a BF weight vector.